Add a README.md file

Signed-off-by: Jacob Kiers <code@kiers.eu>

README.md

@@ -0,0 +1,15 @@
+# bunq-rs
+
+bunq-rs is a Rust wrapper around the [bunq API][0].
+
+It is configured (with [confy][1]) by creating a `bunq-rs.toml` file in
+the OS configuration directory, and with the following contents:
+
+```yaml
+api_key = '<bunq API key>'
+```
+
+The api will take care to update this file with all the requisite information.
+
+[0]: https://doc.bunq.com/
+[1]: https://docs.rs/confy/